A nanomaterial is classified depending upon its size in a particular dimension. If the nanomaterial has three dimensions of less than 100 nm then the nanomaterial can be in the form of a nanoparticle, a quantum dot or a hollow sphere. If the nanomaterial has two dimensions of less than 100 nm then the nanomaterial can be a nanotube, a nanowire or a nanofibre. If the nanomaterial has one dimension less than 100 nm then the nanomaterial will be in the form of a nanofilm or a nanolayer. A nanoparticle may also be referred to as a nanopowder, a nanocluster or a nanocrystal.
Over recent years the study of nanoparticles has received great interest due to unique properties of the nanoparticles. The physical properties of the nanoparticles differ fundamentally from those of the corresponding bulk materials. These different physical properties of the nanoparticles are due to the reduced dimensionality of the nanoparticles which lies between that of a macromolecular substance and that of an atomic substance. The divergence in the physical properties from the bulk material to the nanoparticle material is due to the increase in the ratio of the surface area to volume and the size of the nanoparticle, moving into a realm where quantum effects dominate. The increase in the surface area to volume ratio which is a gradual progression as the nanoparticle gets smaller, leads to an increasing dominance of the behaviour of the atoms on the surface of a nanoparticle over that of the atoms that are in the interior of the nanoparticle.
The quantum effects phenomenon not only affects the properties of the nanoparticle in isolation but also the properties of the nanoparticle during interaction with other materials. Therefore nanoparticles have received much interest in research where large surface area is needed, such as in the fields of catalysis, electrodes, semiconductors, optical devices and fuel cells.
Another feature of nanoparticles is that the nanoparticles provide unique properties that distinguish bulk materials from those of their nanoparticle counterparts. Such unique properties are for example increased strength, increased chemical resistance and increased heat resistance. For example, the bending of copper wire occurs with the movement of copper wire at the 50 nm scale, copper nanoparticles are super hard and do not exhibit the same malleability as the bulk material. A further example is silicon whereby perfectly formed silicon nanospheres with a diameter between 40-100 nm was shown not to be just harder than bulk silicon but its hardness falling between that of sapphire and diamond, therefore making the silicon nanospheres one of the hardest materials known.
Another property of the nanoparticles lies in the fact that once the nanoparticles become small enough, the nanoparticles display quantum mechanical behavior. Such nanoparticles are often referred to as quantum dots or artificial atoms because free electrons within the nanoparticles behave in a manner similar to electrons that are bound to an atom, in that the nanoparticles can occupy certain permitted energy states. Consequently much research is being undertaken on nanoparticles for implementation and use as semiconductors.
A further feature of the nanoparticles is that they have a critical wavelength below that of visible light. The nanoparticles do not scatter visible light, but may also absorb visible light. These absorption properties of the nanoparticles, has seen the nanoparticles employed as a material in applications such as packaging, cosmetics and coatings.
Currently, several methods exist for the manufacture of nanoparticles. Examples of methods for the manufacture of nanoparticles include vapour condensation, chemical synthesis and attrition. A common factor exists in all manufacture methods in that the manufacture parameters are essential for determining the size of the nanoparticles. The manufacture parameters being, for example, temperature, time, and reaction phase. During the manufacture of nanoparticles the manufacture parameters are usually manipulated to provide nanoparticles of a desired size.
The manufacture of nanoparticles by vapour condensation methods, involves the evaporation of a solid material followed by the rapid condensation of the material to form the nanoparticles. Altering the medium into which the vapour is formed affects the size of the manufactured nanoparticles. The evaporation of the solid material and the manufacture of the nanoparticle usually conducted in an inert atmosphere to prevent any possible side reactions, such as the formation of oxides of the materials used. In vapour condensation methods the nanoparticle size is dependant on the apparatus environment and is influenced by the temperature, the gas atmosphere and the rate of evaporation of the material in which the vapour condensation process is conducted. A number of variations of vapour condensation methods exist. A variation being vacuum evaporation on running liquids (VERL). The VERL method utilises a film of viscous material (such as an oil or a polymer) within a rotating drum which is under a vacuum environment. The desired materials are then evaporated into the vacuum whereby they form the desired nanoparticles in a suspension of the viscous material. A further variation of the vapour condensation method is called chemical vapour deposition (CVD). The CVD technique is generally employed in large scale processes for the manufacture of integrated circuits, whereby the nanoparticles are used as semiconductors. In the CVD method, the materials be them liquid or gas are both put in a vaporisation reactor and then subsequently condensed to form the desired nanoparticles.
The chemical synthesis method for the manufacture of the nanoparticles is probably the most popular. The chemical synthesis method allows for low-cost and high volume manufacture of highly mono-disperse nanoparticles. The chemical synthesis method involves the growth of the nanoparticles in a liquid that contains the material reactants. An example of the chemical synthesis method is the sol-gel approach which is often used to manufacture quantum dots. Chemical synthesis methods for the manufacture of nanoparticles are often better than the vapour condensation methods, especially where a certain shape of the nanoparticle is desired. A problem with the chemical synthesis methods for the manufacture of the nanoparticles arises because contamination of the nanoparticles is often observed. The contamination of the nanoparticles is due to precursor substances. Such contamination of the manufactured nanoparticles leads to problems when the nanoparticles are used as surface coatings in sintering methods. Surface coating using sintering methods requires contamination free material to be successful.
Attrition methods for the manufacture of nanoparticles are usually undertaken when the manufacture methods using vapour condensation or chemical synthesis are unsuccessful or when high amounts of low quality nanoparticles with a broad size distribution are needed. Attrition methods utilise the grinding or milling of the material from which the nanoparticle is sought. The milling is usually conducted in a ball mill, a planetary ball mill or other size reducing mechanism. Like the chemical synthesis method for the manufacture of nanoparticles, attrition methods lead to contamination of the nanopartides due the milling material. A further disadvantage is the broad size distribution of the nanoparticle and the limited size range, because in most cases nanoparticles with a diameter smaller than 50 nm cannot be manufactured with attrition methods.
Since the nanoparticles need to be manufactured to a specific size, which leads to their unique properties, they need to be characterised accordingly. Characterisation of the nanoparticles is fundamental to understand and control the manufacture of the nanoparticles. Characterisation of the nanoparticles is usually performed by common analytic techniques such as electron microscopy, atomic force microscopy, x-ray photoelectron spectroscopy, powder x-ray diffractometery, dynamic light scattering and absorption, emission and Fourier Transform infrared spectroscopy.
As the market for nanoparticles continues to rapidly expand due their highly desirable unique properties, the demands for high output, high purity and well defined nanoparticles at low cost expands too. This demand has therefore led to the need for the development of novel manufacturing methods and apparatus for nanoparticles.